Relevant conventional apparatus and methods are exemplified by certain electron-beam projection-exposure apparatus. Projection-exposure using an electron beam is highly accurate but current methods are flawed by low throughput, and various approaches have been investigated in attempts to solve such problems. For example, various "batch"-type projection-exposure systems, termed "cell-projection," "character projection," or "block exposure" systems, have been developed. With such systems, a circuit pattern comprising a large number of repeating units of a particular arrangement of features is defined on a mask. Such a circuit pattern is typical of, for example, memory chips. The entire mask is not exposed in one exposure (or "shot") but rather one unit at a time in a repeating manner onto the substrate. As projected onto the substrate, each unit measures, e.g., about 10 .mu.m by 10 .mu.m square. However, such systems exhibit difficulty in projecting non-repeated areas of the circuit pattern.
Current efforts in the microlithography industry have been directed to the development of electron-beam projection-exposure apparatus that perform "demagnifying" projection-exposure (i.e., exposure in which the projected image of the mask is demagnified relative to the mask) of the mask pattern at much higher throughput than the batch-type systems summarized above. Various approaches have been investigated in which the mask defines the entire circuit pattern for a chip. In one approach, the electron beam is irradiated only on a certain portion of the overall mask pattern at any one instant, and an image of the irradiated portion is demagnified and "transferred" to (i.e., projection-exposed onto) the substrate using a projection lens. If one were to attempt to transfer, using such an apparatus, the entire mask pattern to the substrate in one shot, the entire mask pattern would not be transferred with sufficient accuracy. In addition, preparation of a mask for use in such an apparatus is very difficult.
Therefore, systems that are the subject of the most recent active research are not those in which the entire die pattern (or even multiple die patterns) is exposed in one shot. Rather, the most current approaches are directed to systems in which the projection optical system has a large optical field and the mask pattern is divided into multiple "subfields" that are projection-exposed one subfield at a time onto the substrate. Such systems are termed "divided" projection-exposure systems. With a divided projection-exposure system, exposure of each mask subfield is performed while correcting projection aberrations, such as image focus or field distortion, etc. Thus, projection-exposure can be performed with better image resolution and accuracy across an optically wider field than with systems that projection-expose an entire die in one shot.
In most conventional batch-type and divided projection-exposure systems, the projection-optical system projects a demagnified image of the irradiated portion of the mask onto the substrate. At the mask surface, if all locations of the transverse area of the beam used to illuminate the mask are not incident exactly orthogonally (i.e., telecentrically) at the mask surface, the trajectory of the beam between the mask and the substrate will not be ideal and more aberrations will be manifest.
In addition, at the substrate surface, if all locations of the transverse area of the beam used to project the image onto the substrate surface are not incident exactly orthogonally (i.e., telecentrically) on the substrate surface, problems will arise such as variations in image size as projected and/or image rotation at each transferred subfield, especially if the substrate surface exhibits any variation in elevation relative to the focusing plane of the projection-optical system. Such problems can seriously undermine, for example, the accuracy with which transferred subfields are "stitched" together on the substrate surface.
With semiconductor devices that require highly accurate fabrication, such as 1-Gbit and 4-Gbit DRAMs, the accuracy with which transferred subfields must be positioned and stitched together on the substrate surface is extremely high: about 10 to 30 nm. Therefore, it is becoming increasingly crucial, in such systems, to increase the accuracy with which all portions of the transverse area of the beam are incident orthogonally to the mask and substrate, and to be able to measure such orthogonality with high accuracy.
Conventionally, incidence orthogonality of the center of a beam flux is measured as described in Sturans et al., "Optimization of Variable Axis Immersion Lens For Resolution and Normal Landing," J. Vac. Sci. Technol. B8(6):1682-1685 (Nov./Dec., 1990). According to the method disclosed in that paper, a variable-shaped beam is projected onto the substrate. The size of the imaged pattern is controlled by shaping deflectors. Incidence orthogonality of the beam is measured only at the center of the beam flux. Unfortunately, in divided projection-transfer systems, such corrections cannot be realized.